Apparatus and method for obtaining self-potential logs of boreholes drilled with a non-aqueous drilling fluid



April 9, 1968 J. 5. OSOBA ET AL 3,377,550

APPARATUS AND METHOD FOR OBTAINING SELF-POTENTIAL LOGS OF BOREHOLES DRILLED WITH A NON-AQUEOUS DRILLING FLUID Filed Sept. 14, 1964 2 Sheets-Sheet 1 INVENTORS.

JOSEPH $.0SOBA, 4 BY ROBERTC- RUMBLE,

ATTORNEY.

Apnl 9, 1968 J. s. OSOBA ET AL 3,377,550

APPARATUS AND METHOD FOR OBTAINING SELF-POTENTIAL LOGS OF BOREHOLES DRILLED WITH A NONAQUEOUS DRILLING FLUID Filed Sept. 14, 1964 2 Sheets-Sheet L 6% CABLE SHEATH 'l +5 W53 m M628 56 FIG- 2A. 4

1 W I M II 4 R I BORE-HOLE FLUIDS \J I mux ELECTROLYTE SOLUTION n72 I I INVENTORS. FLOW RATE \1 1 JOSEPH s. OSOBA,

vzL'vE OBERT c- RUMBLE,

FRWSM ATTORNEY United States Patent Filed Sept. 14, 1964, Ser. No. 396,215 4 Claims. (Cl. 324) This invention relates generally to locating earth formations containing hydrocarbon deposits, and more particularly to the logging of wells drilled with non-aqueous drilling fluids.

The technique of obtaining self-potential logs of boreholes drilled with aqueous drilling fluids is well known. This technique is described in detail in the text, The Fundamentals of Electric Log Interpretation, by M. R. J. Wyllie, Academic Press, Inc. (New York, 1957).

When non-aqueous drilling fluids are used in the drilling of boreholes, it has been virtually impossible to obtain a self-potential log of a borehole. Apparatus and techniques previously available have not lent themselves either to the production of self-potential or the measurement of self-potential logs. Unless there is present in a borehole a conducting fluid, no self-potential will be produced to be measured. When a well has been drilled with a non-aqueous drilling fluid, in the past it has been customary to obtain self-potential information from nearby wells drilled with an aqueous drilling fluid. Where no such wells are available, frequently the data to be derived from a self-potential log are considered to be of such importance that the non-aqueous fluid with which the Well was drilled is replaced by an aqueous fluid when the drilling of the well is completed, for the sole purpose of running a self-potential log. Replacing the non-conducting fluid with a conducting aqueous fluid frequently completely nullifies the benefits of using the non-conducting fluid in drilling the well and makes valueless the extra expense involved in using non-conducting drilling fluids. The reason for this is that those formations in Wells Where the non-conducting fluid is most useful are the formations most likely to suffer permeability decreases upon contact with aqueous fluids.

In accordance with the present invention, when a well bore is drilled using the rotary drilling technique with a non-aqueous drilling fluid, there is passed through the well bore a reservoir containing an aqueous electrolyte solution having a known concentration of the electrolyte and a known resistance. Concurrently, there is isolated from the interior of the borehole a portion of the borehole wall proximate to the reservoir, and electrolyte solution from the reservoir is flowed to said isolated portion of the borehole wall. The formation self-potential produced thereby is measured and a record is produced of the measured self-potential as a function of depths in the borehole at which self-potential measurements are taken.

More particularly, there is provided a liquid reservoir adapted to be positioned in a borehole and a pad adapted to be urged against a borehole wall so as to isolate a portion thereof. The pad is carried on a mechanism adapted to hold the pad away from the borehole wall until the reservoir is at a desired level in the borehole, and then to urge the pad against the borehole wall. A conduit interconnecting the reservoir and the pad is provided with a valve for preventing fluid flow therethrough until the pad is so urged against the borebhole wall. Preferably, there is also provided in the conduit a valve for regulating the fluid flow through the conduit so that the fluid flows at substantially a constant flow rate. Electric circuit means is provided for measuring the electrical selfpotential generated when fluid is flowed from the reservoir to the face of the pad.

Objects and features of the invention not apparent from the above discussion will become evident upon consideration of the following detailed description thereof taken in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of apparatus for measuring self-potential in accordance with the invention;

FIGS. 2A and 2B are cross-sectional side views of a preferred apparatus in accordance with the invention, said apparatus being shown positioned in a borehole being logged;

FIG. 3 is a front view of the pad 20 illustrated in FIGS. 1 and 2B;

FIG. 4 is an enlarged cross-sectional view of a portion of the apparatus of FIG. 1;

FIG. 5 is an enlarged cross-sectional view showing the hold-down means for releasably securing the pad in the retracted position so that the apparatus can be lowered in a borehole; and

FIG. 6 is a bottom view of the device shown in FIG. 5.

In FIG. 1 there is shown a logging sonde 11 suspended in a borehole by means of logging cable 12 from a sheave 80. The logging sonde 11 is provided with pads 18 and 20 which are shown as being rammed or urged against the wall of borehole 94. The logging cable 12 is provided with an electrical lead which is connected to a self-potential measuring device 82 for measuring variations in the self-potential between pad 20 and a ground electrode 92. The self-potential measuring device 82 may include a recording voltmeter 84 electrically connected to a tap of potentiometer 88 which is electrically connected in parallel with a battery 86. The negative terminal of battery 86 is connected to ground electrode 92. The tap 88 is adjusted so that the variations in potential measured 'by recording voltometer 84 are those produced by the electrical self-potential between the formation being logged and the pad 20.

The constructional details of the sonde 11 are illustrated in FIGS. 2A6. The sonde comprises an upper section 11A containing a reservoir 37 for an electrolyte solution having a known concentration and a known resistance. The sonde also includes a lower portion 16 which essentially is a portion of a conventional microlog sonde of the type previously employed by commercial logging companies. The sonde 11 is supported by cable sheath 12 and is electrically insulated therefrom by insulating member 14. Insulated electrical conductor 13 of the cable is grounded to the sonde body, shown in FIG. 2A at point 15.

The lower section 16 of the sonde includes bow springs 17 and 19 attached at their upper extremities to pivot pins 56 and 57, respectively. The body of the lower portion 16 comprises cylindrical members 31 and 72 screw threadedly attached to I-beam 53 (the threads are not shown). At their lower extremity members 17 and 19 are attached to pivot pins 58 and 59 mounted in a movable cylindrical member 10 whose free axial movement in cylindrical member 72 is limited by studs 61 and 62 attached to cup member 60 and moving within slots 63 and 64, respectively, in member 72.

Attached to the lower end of cup member 60 are thin metal rods 65 and 66 (see FIGS. and 6), terminated at their lower ends by threadedly-engaged and screw driver-slotted tapered caps 67 and 68. Rods 65 and 66 protrude through holes 69 and 70 of metallic wall member 71 which is formed as a part of cylindrical member 72. Holes 69 and 70 are of suflicient diameter to allow for ready passage of cap members 67 and 68. Wall member 71 also incorporates a centrally disposed, dead-ended cylindrical hole opening downwardly. Passing through the middle of the closed end of this hole is the lower end of insulated conductor 47 which conducts firing current to a filament housed in encapsulated firing squib 46. The firing squib is urged upwardly by spring 74 which bears downward on aluminum sealing member 75 and acts as an electrical conductor between the lower terminus of the firing squib filament and the grounded member 75. Member 75 incorporates two sealing O-rings around its section of reduced diameter and is of such diameter at its lower end that it partialy blocks holes 69 and 70 of wall member 71 when in the position shown in FIGS. 5 and 6. This blockage prevents free pasage of cap members 67 and 68 through these holes as long as sealing member 75 remains in the position shown. The tapered contour of cap members 67 and 68 exerts an upward force component on sealing member 75 when rods 65 and 66 are in tension. Pads 18 and 20 are affixed to bow springs 17 and 19, and an additional pad 21 is vulcanized to pad 20 on its exterior face. A fluid line or passage 22, which will be described below, is inserted or formed through the middle of pads 20 and 21. As shown most perspicuously in FIG. 3, the pad 21 comprises a plurality of raised ridges 52 which are also concentric about conduit 22. The pads 18, 20, and 21 preferably are of rubber or a rubber-like material. The ridges 52 preferably are between and inch in depth.

As shown in FIG. 6, sealing member 75 has peripheral notches 76 therein and a shallow screw driver slot 77 to facilitate the placement of member 75 in the position shown in FIGS. 5 and 6. In practice, the following procedure is useful for locking the pads of the microlog sonde in a retracted position. Into the centrally disposed hole in wall member 71 are introducedfirst, firing squib 46, then spiral spring 74, followed by sealing member 75 with its peripheral notches oriented to clear holes 69 and 70 of wall member 71. Sealing member 75 will not drop out of member 71 because of friction provided by its O-rings. The bow springs 17 and 19 and pad members are then forced inwardly toward I-beam member 53. This action will cause tapered caps 67 and 68 to move downwardly through holes 69 and 70 so that by rotation of member 75 by means of screw driver slot 77, these cap member will be pushed radially outward so that they cannot move upwardly through holes 69 and 70 as long as member 75 remains in place. Rods 65 and 66 are placed in tension by tightening caps 67 and 68 utilizing the screw driver slots on their lower ends. If bow springs and pad members 17 and 19 are released they will remain in a retracted position until the firing of squib 46 by current flowing in line 47 forces sealing member 75 downward into the lower end of the sonde. Rod members 65 and 66 are then free to move upward and allow the preformed bow spring to spring outwardly to force the pads against the walls of the borehole.

While the apparatus described above is preferred for carrying the pads into a well bore and for forcing the pads against the walls of a borehole at a predetermined depth therein, other apparatus known to the art may be used for this purpose, such as that described in U.S. Patent 2,899,633-O. R. Smith et a1.

As shown in FIG. 2A, the sonde housing 11A houses a differential piston arrangement consisting in part of a tubular, annular member 24 extending upwardly from wall member 30 toward the upper end of the housing. The tubular, annular member or pipe 24 supports a piston or sealing member 23 which provides a seal between pipe 24 and the interior wall of open-ended cylinder 25 by means of O-ring 26. An annular movable piston 27, sealing against the inner wall 28 of the lower section of sonde housing 11A by means of O-ring 29, is adapted to move between the wall member 30 and a predetermined upper position in the housing determined by stop 31. Mounted on piston 27 is a domed, annular member or pipe 32 closed on its upper end but all-owing for passage ,of insulated conductor 33 which contacts the cable sheath 12 through an intervening quick-disconnect 34. Open ended cylinder 25 is also attached to piston 27 concentric with and spaced from domed member 32 as shown. In the wall of body member 11 above the position of piston stop 31 are two ports or windows 35 and 36 which permit the entrance of drilling fluid to actuate the movable piston27. Fluid reservoir 37 inside member 11 and below movable piston 27 is filled with electrolyte solution of predetermined concentration and resistance by means of plug 38 in the wall of cylinder 11. The concentration of the electrolyte solution is not particularly critical so long as it is known, along with'its resistance, and can be used over an extremely broad range. In the electrolyte solution, the preferred solute is NaCl. Alternatively, a solute having an ion in common with the most prevalent ion in the formation fluids can be used, such as water-soluble chlorides and souble sodium compounds.

Fluid passageway 39 in wall member 30 is covered on its upper end by filter member 40 and on its lower end connected to a constant flow rate valve 41, which permits a substantially uniform rate of liquid to flow therethrough over widely divergent pressure ranges. Valve 41 preferably is of the type described in U.S. Patents No. 2,389,134 and No. 2,454,929, and which is marketed under the trade name of Dole Flow Control Valve, Dole Valve Company, Martingrove, Ill.

Valve 41 is attached to and may form an integral part of flexible fluid conduit 42 which leads to fluid shut-off valve 43. This valve is actuated by rod 44, which in turn is attached at its lower end to the movable cylindrical member 10. When the bow springs are in the position shown in FIG. 2B, valve 43 allows free passage of fluid. However, when the springs are in the retracted position, valve 43 is closed and fluid flow is cut off. Flexible conduit 45 completes the fluid path from valve 43 to the exterior opening of fluid line 22 through pad 20.

FIG. 4 illustrates how a telescoping contact is maintained between conductors 33 and 47 of FIG. 2. Insulated conductor 33 passes through domed pipe 32, as seen in FIG. 2A, and then through slim tubular member 48 to contact wiper arm 49 on the lower end of tubular member 48. Wiper arm 49 electrically contacts the interior wall of thin brass cylinder 50 mounted concentrically within stationary piston rod 24 and separated therefrom by insulating sleeves 51. The lower end of brass cylinder 50 attaches to the upper end of conductor 47 leading to firing squib 46.

The apparatus described above is lowered into the wells with bow spring retracted. At the desired depth, lead 33 is electrically energized to fire squib 46. Bow springs 17 are thereupon released to open valve 43 and start the flow of electrolyte solution from the pad. The sonde is slowly pulled upwardly through the hole so that between one and one and one-half gallons of the electrolyte solution is flowed against the filter cake per linear feet of the borehole. When the squib is fired, the electrical connection between the cable sheath and the sonde body is broken inasmuch as the squib-to-sonde body connection will be interrupted. Further interruption of this circuit occurs by quick-disconnect 34 as the piston moves downwardly. This leaves the body of the sonde tied to the inner conductor of the cable, shown at point 15. The sonde is electrically connected to the interstitial water in the formation through the electrolyte solution in the sonde and the conduit leading from the reservoir 37 to the face of pad 21. If desired, conduits 42 and 45, wall member 30, and valves 41 and 43 may be made of electrically conductive material to increase the electrical conductivity of the circuit. As the sonde is raised through the borehole, the recording voltmeter 84 will record variations in the self-potential produced by the difference between the concentration of ions in the interstitial waters and the electrolyte solution at the face of the sonde. The variations in the self-potential are plotted as a function of depth by conventional means to give a self-potential log of the formations traversed by the sonde.

It is well known that the salt content of the waters in earth formations increases with increasing depth, and that at the temperature of the deeper formations, the waters often contain more than saturated solutions when expressed at surface conditions. This means that it is possible for the logging sonde to traverse some formations that contain fluids with less concentration of salts and other formations that contain fluids more highly concentrated with salts when compared with any solution that might be placed in the reservoir of the sonde. Thus, at some point in the well there is likely to be a stratum that contains water with a salt concentration equal to that of the fluid in the sonde reservoir. The self-potential at that point will be zero. This point in the Well can be recognized and can serve a useful purpose in checking the values of resistivity of the formation fluids as computed from the self-potential log using standard techniques. At depths above this point of equal concentrations, the self-potential will be designated by one algebraic sign; at this particular point the self-potential will be zero; and at depths below this point (where the salt concentration is greater) the self-potential will be designated by the opposite algebraic sign.

The invention is not necessarily to be restricted to the specific structural details, arrangement of parts, or circuit connections herein set forth, as various modifications thereof may be effected without departing from the true spirit and scope of the invention.

What is claimed is:

1. A method of producing a self-potential log for locating a hydrocarbon productive earth formation comprising the steps of:

drilling a borehole using the rotary drilling technique with a non-aqueous drilling fluid;

continuously moving through the borehole a reservoir containing an electrolyte solution;

While said reservoir is moving concurrently isolating from the interior of the borehole a portion of the borehole wall proximate to the reservoir;

concurrently flowing said electrolyte solution at a constant flow rate from the reservoir to the isolated portion of the borehole wall and measuring the formation self-potential produced thereby; and

producing a record of the measured self-potential as a function of depths in the borehole at which self-potential measurements are taken.

2. A method of producing a self-potential log for cating a hydrocarbon productive earth formation comprising the steps of:

drilling a borehole using the rotary drilling technique with a non-aqueous drilling fluid;

continuously moving through the borehole a reservoir containing an electrolyte solution;

while said reservoir is moving concurrently isolating from the interior of the borehole a portion of the borehole wall proximate to the reservoir;

concurrently flowing said electrolyte solution from the reservoir to the isolated portion of the borehole wall at a constant flow rate such that between one and one and one-half gallons of electrolyte solution flow from the reservoir per linear feet of the borehole;

concurrently measuring the formation self-potential produced thereby; and

producing a record of the measured self-potential as a function of depths in the borehole at which self-potential measurements are taken.

3. Apparatus for conducting a continuous self-potential log of a borehole, comprising:

an elongated housing;

a pad of rubber or rubber-like material connected to said housing, having a face comprising a plurality of circular ridges of successively increasing diameter for contacting the walls of the borehole, said pad substantially conforming to the contour of the borehole wall;

support means connected to said housing having a retracted position and a normal extended position for urging said pad into engagement with said borehole wall when in said extended position;

a reservoir for an electrolyte solution in said housing above said pad;

a conduit extending from said reservoir through the proximate center of said pad;

a constant flow rate valve in said conduit for controlling the flow rate of liquids from said reservoir through said conduit;

a normally-closed valve in said conduit;

means connected to said support means and to said normally-closed valve for opening said normallyclosed valve responsive to movement of said support means from said retracted position to said extended position;

electrically conductive means for establishing an electrical circuit from the earths surface through said housing to the face of said pad; and

electrical measuring means connected to said electrically conductive means and for connection to the earth at the earths surface, for measuring the selfpotential of any earth formation contacted by said pad.

4. Apparatus for conducting a continuous self-potential log of a borehole, comprising:

an elongated housing defining a chamber having a normally upper and a normally lower end;

a first tubular member extending into said chamber from said lower end thereof;

an annular piston surrounding said first tubular member and slidingly engaging the inner wall of said housing adapted to move in said chamber between a given position therein toward the lower end thereof;

means including a domed tubular member positioned around said first tubular member and affixed to said annular piston and extending therefrom toward the upper end of said elongated housing over said first tubular member;

sealing means between said first tubular member and said domed tubular member for permitting relative movement therebetween so as to isolate the interiors thereof;

said housing being ported above said given position of said piston to provide fluid communication between said chamber and the surrounding borehole;

a pad for contacting the wall of the borehole and isolating a portion thereof from the interior of the borehole;

means aflixed to said housing and to said pad for mov- 7 ing said pad between a retracted-position and a position whereat it is engaged with the borehole wall; fluid conduit means between said chamber and said pad for conducting fluid from said chamber to the portion of the borehole wall isolated by said pad; electrically conductive means for establishing an electrical circuit from the earths surface through said housing to the face of said pad; and electrical measuring means connected to said electrically conductive means and for connection to the earth at the earths surface, for measuring the selfpotential of any earth formation contacted by said pad.

References Cited UNITED STATES PATENTS Doll 324-10 X Doll 324-10X Graham et al. 324--1 Smith et a1. 324-l0 Blanchard 23230 Terry 324-10 Vogel 32410 X Oliver 324-1 RUDOLPH V. ROLINEC, Primary Examiner.

G. R. STRECKER, Assistant Examiner. 

1. A METHOD OF PRODUCING A SELF-POTENTIAL LOG FOR LOCATING A HYDROCARBON PRODUCTIVE EARTH FORMATION COMPRISING THE STEPS OF: DRILLING A BOREHOLE USING THE ROTARY DRILLING TECHNIQUE WITH A NON-AQUEOUS DRILLING FLUID; CONTINUOUSLY MOVING THROUGH THE BOREHOLE A RESERVOIR CONTAINING AN ELECTROLYTE SOLUTION; WHILE SAID RESERVOIR IS MOVING CONCURRENTLY ISOLATING FROM THE INTERIOR OF THE BOREHOLE A PORTION OF THE BOREHOLE WALL PROXIMATE TO THE RESERVOIR; CONCURRENTLY FLOWING SAID ELECTROLYTE SOLUTION AT A CONSTANT FLOW RATE FROM THE RESERVOIR TO THE ISOLATED PORTION OF THE BOREHOLE WALL AND MEASURING THE FORMATION SELF-POTENTIAL PRODUCED THEREBY; AND PRODUCING A RECORD OF THE MEASURED SELF-POTENTIAL AS A FUNCTION OF DEPTHS IN THE BOREHOLE AT WHICH SELF-POTENTIAL MEASUREMENTS ARE TAKEN. 